In color or full-color organic electroluminescent (EL) displays, there is an array of colored pixels such as red, green, and blue color pixels (commonly referred to as RGB pixels). Precision patterning of the color-producing organic EL media is required to produce the RGB pixels. The basic organic EL device has in common an anode, a cathode, and an organic EL medium sandwiched between the anode and the cathode. The organic EL medium can consist of one or more layers of organic thin films, where one of the layers is primarily responsible for light generation or electroluminescence. This particular layer is generally referred to as the light-emitting layer of the organic EL medium. Other organic layers present in the organic EL medium can provide electronic transport functions primarily, such as the hole-transporting layer or the electron-transporting layer. In forming the RGB pixels in a full-color organic EL display panel, it is necessary to devise a method to precisely pattern the light-emitting layer of the organic EL medium or the entire organic EL medium.
Typically, electroluminescent pixels are formed on the display by shadow masking techniques, such as shown in U.S. Pat. No. 5,742,129. Although this has been effective, it has several drawbacks. There are problems of alignment between the substrate and the shadow mask, and care must be taken that pixels are formed in the appropriate locations. When it is desirable to increase the substrate size, it is difficult to manipulate the shadow mask to form appropriately positioned pixels. A further disadvantage of the shadow-mask method is that the mask holes can become plugged with time. Plugged holes on the mask lead to the undesirable result of non-functioning pixels on the EL display.
There are further problems with the shadow mask method, which become especially apparent when making EL devices with dimensions of more than a few inches on a side. It is extremely difficult to manufacture larger shadow masks with the required precision for accurately forming EL devices.
A method for patterning high-resolution organic EL displays has been disclosed in U.S. Pat. No. 5,851,709 by Grande et al. This method is comprised of the following sequences of steps: 1) providing a donor substrate having opposing first and second surfaces; 2) forming a light-transmissive, heat-insulating layer over the first surface of the substrate; 3) forming a light-absorbing layer over the heat-insulating layer; 4) providing the substrate with an array of openings extending from the second surface to the heat-insulating layer; 5) providing a transferable, color-forming, organic donor layer formed on the light-absorbing layer; 6) precision aligning the donor substrate with the display substrate in an oriented relationship between the openings in the substrate and the corresponding color pixels on the device; and 7) employing a source of radiation for producing sufficient heat at the light-absorbing layer over the openings to cause the transfer of the organic layer on the donor substrate to the display substrate. A problem with the Grande et al. approach is that patterning of an array of openings on the donor substrate is required. This creates many of the same problems as the shadow-mask method, including the requirement for precision mechanical alignment between the donor substrate and the display substrate. A further problem is that the donor pattern is fixed and cannot be changed readily.
Using an unpatterned donor sheet and a precision light source, such as a laser, can remove some of the difficulties seen with a patterned donor. Such a method is disclosed by Littman in U.S. Pat. No. 5,688,551, and in a series of patents by Wolk et al. (U.S. Pat. Nos. 6,114,088; 6,140,009; 6,214,520; and 6,221,553). The latter patents teach a method that can transfer, by a change in adhesion, the light-emitting layer of an EL device from a donor sheet to a substrate by heating selected portions of the donor with a scanning laser light spot. While this is a useful technique, there are serious difficulties in applying it on a large-scale manufacturing of EL devices. To make an EL device that includes thousands—or even millions—of pixels in three colors in a reasonable amount of time (a few minutes) would require a laser spot that moves very fast in two dimensions. The need for rapidly moving machinery increases the demands in terms of dynamic structural stability. A failure to control the alignment between the laser source and the substrate due to machine vibrations will result in a decrease in display quality. A further disadvantage is that the rapid movement of the laser beam necessitates a very short dwell time on each spot to be transferred, which further necessitates a very high-powered laser.
Another method for patterning high-resolution organic EL displays has been disclosed in U.S. Pat. No. 6,582,875 by Kay et al. and uses a multichannel laser thermal printhead with a donor and a receiver. The printhead is first aligned to the receiver, and the printhead then scans and exposes the donor in the regions desired for donor transfer to the receiver. Many channels allow an increase in dwell time at each donor pixel site, while maintaining productivity. However, devices made in a scanning mode often do not perform as well as standard evaporated devices.